1. Field of the Invention
This invention relates to a method for producing K.sub.2 SO.sub.4 crystals comprising reacting a potassium chloride salt, a calcium sulfate salt and a sulfate source to produce syngenite, decomposing the syngenite with a mineral acid to produce KHSO.sub.4, crystallizing the KHSO.sub.4 to produce the double salt crystal K.sub.3 H(SO.sub.4).sub.2 and recrystallizing the thus-formed K.sub.3 H(SO.sub.4).sub.2 crystals to recover K.sub.2 SO.sub.4 in crystalline form.
2. Description of the Prior Art
Potassium chloride, the major form in which potassium is used in fertilizers, has been known for many years to have agronomic disadvantages when compared with certain other potassium salts. Thus, currently the sulphate and nitrate are widely used on crops such as tobacco, tomatoes, and potatoes, especially for those to be used in the production of potato chips (crisps).
The chloride ion, if allowed to build to sufficiently high levels, is toxic to most plant species, and its elimination is a desirable aim for the fertilizer industry. In arid areas, totally dependent upon irrigation for their water, for example, the build-up of chloride ions in the soil can become a major factor in producing a reduced crop yield. At such times, major quantities of water are required to flush out the chloride. Such flushing not only wastes large quantities of valuable water, but, at the same time, flushes out necessary fertilizer constituents in the soil.
The major, if not the sole, factor which has caused the continued use of potassium chloride under these circumstances, is the ready availability and consequent low cost of the chloride as compared with other potassium salts.
The most common substitute for potassium chloride is potassium sulphate. This salt exists in various mineral forms in a number of places, but its separation, usually by crystallization techniques, is more complex and more expensive than that for the chloride. It is produced directly as a double sulphate salt along with magnesium sulphate in the Western U.S.A., but such material, although not expensive, per se, is low in potassium concentration and hence more costly to transport and store.
For many years, the chloride salt of potassium has been converted into the sulphate by high temperature reaction with sulfuric acid, and considerable quantities are manufactured in this way, particularly in Belgium. U.S. Pat. No. 4,342,737 discloses one such process. The major factors restricting further production by this method are threefold:
1. The high energy requirement.
2. The highly corrosive nature of the reactants and the by-product hydrogen chloride.
3. The need for a local market for the hydrogen chloride produced--otherwise, it must be neutralized at considerable cost before it can be discarded.
For many years, varying routes have been described to convert the chloride using calcium sulphate or sodium sulphate, but none has been used commercially up to the present.
Many of the routes proposed produce glaserite, a double salt of potassium and sodium sulphate, Na.sub.2 SO.sub.4.3K.sub.2 SO.sub.4, as an intermediate and subsequently react with excess potassium chloride to convert the sodium sulphate to potassium sulphate. The product may be recovered, for example, by evaporation and recrystallization. U.S. Pat. No. 4,215,100 is directed to such a process. Other routes produce the calcium double salt, syngenite, CaSO.sub.4.K.sub.2 SO.sub.4.H.sub.2 O as intermediary. This may be decomposed by water at elevated temperature and pressure, as disclosed in British Patent No. 435,772, or by concentrated ammonia at low temperature, as disclosed in French Patent No. 787,713. In British Patent No. 2,068,918, sylvinite, a double salt of potassium and sodium chloride of variable composition, and calcium sulphate are reacted with aqueous ammonia to produce the potassium sulphate, sodium sulphate double salt; the double salt is reacted with sylvite or additional sylvinite in aqueous ammonia to produce potassium sulfate crystals.
All such routes are complex, costly, major energy users and may require operation under undesirable conditions.
Thus a need has continued to exist for a process of producing K.sub.2 SO.sub.4 using readily available raw materials of low cost, said process being relatively uncomplicated, highly energy efficient, and requiring no substantial equipment cost outlay.